The present invention relates to stators, for dynamoelectric machines, which are particularly useful for high air-gap magnetic flux conditions, such as those produced in the use of superconducting rotors.
Generators are dynamoelectric machines generally possessing a rotor and a stator. In the typical large generator employed by electric utilities to produce electricity, a rotor is driven by a steam turbine or other source of motive force so as to produce rotational movement of the rotor within the stator assembly. The rotor is conventionally provided with field windings which are connected to energizing and excitation circuitry through the use of slip rings and carbon brushes or other electromagnetic coupling means. The rotor provides a rotating magnetic field which is radially directed with respect to the generally cylindrical rotor assembly. Because of the rotary motion, the rotor magnetic flux field is made to interact with stationary bars of insulated conductor material in the stator assembly so as to induce electrical energy in the stator winding bars. Electrical connections to the bars in the stator winding couple the generator to the typical three-phase electrical power system of most electric utilities. Various rotor and stator constructions are conventionally known in the art.
Because of increasing fuel prices, the efficiency of the turbine-generator combination has become an increasingly important parameter of interest. Increases in overall system efficiency are looked upon favorably. One method of increasing generator efficiency is through the use of superconducting rotors, that is, rotors having windings which are cooled to a temperature below which they are superconducting; that is to say, that at these lower temperatures the resistance of the conductive material employed in the rotor or field windings drops to almost zero. Because superconducting windings exhibit an extremely low value of resistance at these cryogenic temperatures, the ohmic looses in the rotor winding are almost negligible. Accordingly, undesirable thermal losses in the rotor do not occur and the overall efficiency of the machine is increased. Moreover, the level of magnetic flux provided may be significantly increased.
While the use of superconducting materials in the rotor winding provides benefits in terms of thermal and cost efficiency, nonetheless various problems arise because of the high levels of magnetic flux density present in the vicinity of the gap between the stator and the rotor. Since magnetic flux densities of up to 2 tesla may occur in such machines, it is desirable to design the stator winding support system using nonmetallic material rather than the iron teeth used in conventional design practice. The nonmetallic support system must hold the stator bars which are subjected to vibratory tangential and radial forces during normal operating conditions. During short-circuit conditions radial forces may be two hundred times greater and tangential forces twenty times greater than the forces acting on the stator bars during normal operation. It is also known that the stator core itself experiences an oscillating ellipticity during operating conditions and a greater ellipticity during short-circuit loads. The forces arising because of the electromagnetically induced reaction also tend to produce relative motion between the various portions of the stator core including the stator bars themselves. Moreover, during startups, shutdowns and load changes, thermal gradients result which cause relative movement of the winding bars within the stator slots due to thermal expansion forces. Additionally, during unusual conditions such as sudden short circuits, torques as high as eight to twenty times higher than normal can result. The stator support system must be able to transmit all of the torque developed on the stator winding bars to the core structure and eventually through the surrounding generator frame to the power station foundation itself. Additionally, it is to be noted that all of these thermal and electromagnetic forces are present in a high-voltage, high-current electrical environment and must also provide proper means of supplying electrical insulation and grounding protection. In particular, a stator bar winding support system should include a means for adequately providing ground circuit paths between the insulation on the bar windings and the frame and core assembly. All of these requirements must be met under conditions which require a large cross-sectional area for the stator windings. These stator bar windings are generally copper and are cooled separately from the rotor windings. The annular space occupied by these copper stator bars is to be maximal within the constraints imposed by the necessity of providing adequate support against the possibility of relative motion between the stator bar windings and the support structures.
Although not specifically identified as a structure useful in superconducting applications, there is apparently disclosed in U.S. Pat. No. 4,179,635 issued Dec. 18, 1979 to Heinrich Beermann, a support structure comprising an outer lamination stack in the center of which there is a hollow cylindrical nonmagnetic holding member and a further hollow inner cylinder of synthetic material which is held in place with wedges and which holds the stator bars in place in slots along the inner periphery of the cylindrical holding member. Such a structure is illustrated in FIG. 1 of the Beermann patent. The holding member is further described as comprising a nonmagnetic stainless steel. Unfortunately, this structure exhibits certain deficiencies. A particular weakness is found in the vicinity between the outer core laminations and the inner holding member. In particular, there does not appear to be any method for holding these two structures together to prevent relative rotational motion between them, such as might be induced in normal operation and by sudden short circuits, startup or other abnormal line conditions. Furthermore, the structure illustrated by the Beermann patent requires an additional structure acting to retain the stator bar windings within the stator slots. This structure unfortunately acts in a deleterious way in unnecessarily spacing the stator windings at a greater distance from the rotor than is necessary. Additionally, the holding member and the outer stack of laminations do not form an integral structure. Because of the electromagnetically induced ellipticity in the stator core, disadvantageous stresses are placed upon the holding member. Also, the holding member, being composed of stainless steel will have eddycurrent losses which reduce the efficiency of the generator.
Others have also apparently developed structures for supporting the stator winding bars in dynamoelectric machines. Examples of such structures are to be found in U.S. Pat. No. 4,068,142 issued Jan. 10, 1978 to Gillet et al., in U.S. Pat. No. 3,405,297 issued Oct. 8, 1968 to Madsen et al., in U.S. Pat. No. 3,743,867 issued July 3, 1973 to Smith, and in British Patent Specification No. 1,365,191 published Aug. 29, 1974 in the names of Preston et al. These patents generally recognize the desirability of increasing the cross-sectional area of the stator bar winding in the vicinity of the air gap. Madsen et al. are particularly cognizant of the significantly greater short-circuiting forces which can result.